Figure 1.
Action potentials of TRN neurons exhibit similarly large threshold voltage range during different network oscillations.
A, C, In vivo intracellular recordings (intra) from two TRN neurons during either slow oscillations (ketamine-xylazine anaesthesia), A) or spindle oscillations (pentobarbital anaesthesia), C) recorded in the neocortex. B, D, Superposition of all action potentials (clipped) depicted on the left column (A, n = 61; C, n = 44) showing the variability in voltage threshold. Horizontal line shows mean voltage threshold for the whole recording epoch (B, −47.7 mV; D, −52.3 mV). Spikes were arbitrarily clipped for illustration purposes only. E, Distribution of spike thresholds for neurons shown in A and C (slow oscillations, n = 381 spikes; spindle oscillations, n = 633 spikes). F, Action potential voltage threshold range for all recorded neurons (slow waves, ketamine-xylazine anaesthesia; spindles, pentobarbital anaesthesia). Circles and grey bars show individual values and means, respectively. Means were not significantly different (p = 0.9089). Intracellular recording (intra-TRN, 0.1 Hz–20 kHz), cortical local field potential (LFP-cx, 0.1 Hz–20 kHz), slow waves (filtered LFP, 0.1–2 Hz), spindles (filtered LFP, 7–15 Hz). Scale bars: A,C, LFP-cx 0.5 mV, slow waves 0.5 mV, spindles 0.5 mV, intra-TRN 20 mV, horizontal 1 s; B,D, vertical 5 mV, horizontal 0.2 ms. Some portions of raw data (intracellular voltage recordings) from the cells in this figure have been used in previous publications (see Methods).
Figure 2.
The action potential threshold range is an intrinsic membrane property.
In vivo firing patterns of a TRN neuron (ketamine-xylazine anaesthesia) intracellularly recorded during spontaneous synaptic active activity (A, active network) or intracellular current injection (B, current pulse, +1 nA). C, Distribution of voltage thresholds for the previous neuron during both conditions (active network, n = 1404 spikes; current pulses, n = 330 spikes). Note different scales for both conditions. D, Action potential threshold range for TRN neurons driven by intracellular current pulses or spontaneous active network episodes. Means were not significantly different (n = 5, p = 0.248). Circles and grey bars show individual values and means, respectively. Scale bars: A, B, vertical 20 mV, horizontal 50 ms.
Figure 3.
The voltage threshold of action potentials is negatively correlated with the preceding rate of membrane potential depolarization.
A, Three action potentials (clipped) recorded from a TRN neuron are shown with the membrane potential change leading to the action potential onset. Each recording is shown with its respective best-fit line prior (10 ms) to the action potential onset. Spikes were arbitrarily clipped for illustration purposes only. B, Plot of the action potential voltage threshold versus the membrane potential change over the 10 ms preceding each action potential (pre-spike Vm slope) recorded in the cell shown in A. The line is the best-fit line by the equation y(V) = −51.34 mV – 2.09 ms*x(dV/dt), n = 405 spikes, R = −0.2497, p<0.0001, two-tailed t test. C, Subthreshold EPSPs triggered by electrical stimulation of a topographically connected cortical area. Note the long-lasting decay of the EPSPs. D, Suprathreshold EPSPs followed by action potentials with different latencies in the same cell shown in C. Spikes were arbitrarily clipped for illustration purposes only. E, Three action potentials (clipped) triggered by evoked EPSPs are shown with the membrane potential change leading to the action potential onset. Each recording is shown with its respective best-fit line prior (1 ms) to the action potential onset from the cell shown in C. Spikes were arbitrarily clipped for illustration purposes only. F, Plot of the action potential voltage threshold versus the membrane potential change over 1 ms preceding each action potential (pre-spike Vm slope) recorded in the cell shown in C. The line is the best-fit line by the equation y(V) = −44.68 mV – 0.62 ms*x(dV/dt), n = 77 spikes, R = −0.5728, p<0.0001, two-tailed t test. Scale bars: A, C,D, vertical 5 mV, horizontal 2 ms; E, vertical 10 mV, horizontal 1 ms. Some portions of raw data (intracellular voltage recordings) from the cells in this figure have been used in previous publications (see Methods).
Figure 4.
Action potential voltage threshold is negatively correlated with the maximal rate of depolarization during the action potential.
A, Three different action potentials during the rising phase. Vertical arrows depict the position of the threshold (left) and maximal slope (right). Inset, derivative of the action potential at the point indicated by the arrow. Note different time scale for the inset. B, Plot of the action potential maximal rising slope versus voltage threshold for all recorded action potentials (n = 633 spikes). The line is the best-fit line by the equation y(dV/dt) = −105.72 mV/ms – 7.38 ms−1*x(V), R = −0.725, p<0.0001, two-tailed t test. C, bottom, sequence of action potentials discharged by the cell shown in A. Top, derivative (dV/dt) of the membrane potential (intra-TRN) for the trace at the bottom. Note variation in peak amplitude in the derivative. D, Plot of the action potential maximal rising slope versus the inter-spike interval for all recorded action potentials (n = 633 spikes). The line is the best-fit line by the exponential function y(dV/dt) = 327.43 mV/ms+68.5 mV/ms*exp (x(V)/164.66 ms), p<0.05, Kolmogorov-Smirnov test. Scale bars: A, spike, vertical 20 mV, horizontal 0.1 ms; derivative, vertical 50 mV/ms, horizontal 0.1 ms; C, dV/dt, vertical 200 mV/ms, horizontal 20 ms; intra-TRN, vertical 50 mV, horizontal 20 ms. Some portions of raw data (intracellular voltage recordings) from the cell in this figure have been used in previous publications (see Methods).
Figure 5.
The voltage threshold of action potentials is a function of the preceding spike time interval.
A, Plot of the action potential voltage threshold versus the time (in logarithmic scale) since the last action potential recorded in a TRN neuron. The line is the best-fit line by the log-normal function y(V) = 48.33 mV+7.03 mV*exp[−(ln(x(t)/23.52 ms)/1.64)2], n = 3644 spikes, p<0.001, Kolmogorov-Smirnov test. B–D, Three interval ranges in the log-normal distribution correspond to discrete physiological intracellular states of activity. B, high-frequency burst discharge; C, inter-burst discharge periods; and D, inter-network active periods. Grey horizontal lines show for each spike the voltage threshold. Scale bars: B, vertical 20 mV, horizontal 20 ms; C, vertical 20 mV, horizontal 100 ms; D, vertical 20 mV, horizontal 1 s. Some portions of raw data (intracellular voltage recordings) from the cell in this figure have been used in previous publications (see Methods).
Figure 6.
The action potential threshold depends on the occurrence of multiple previous action potentials.
A, In vivo intracellular recording of a TRN neuron during slow oscillations in the cortex (ketamine-xylazine anaesthesia) shows the large range of inter-spike intervals. B, A segment from A (horizontal bar) is expanded for clarity. For this cell, it was found that the threshold of any particular action potential (arrow) is influenced by eight previous ISIs (grey bars). C, Summary results for all TRN neurons recorded for this study. Plot of the number of significant partial regression steps (p<0.05) versus the mean ISI for each neuron. Line is the best-fit line by the equation y = −7.96–0.02*x(t), r = −0.8481, n = 23 cells, p<0.0001, two-tailed t test. Scale bars: A, LFP-cx 1 mV, intra-TRN 10 mV, 0.5 s; B, vertical 20 mV, horizontal 100 ms.
Figure 7.
The onset speed of action potentials is voltage-dependent.
A, In vivo intracellular recording of a TRN neuron during spindle oscillations (pentobarbital anaesthesia). Inset, typical high-frequency burst discharge. B, Sample of consecutive action potentials (n = 10) from the cell shown in A. Inset, higher magnification of the action potential onset shows the absence of kink C, Derivative of the action potentials shown in B. Note different time scale than B. D, Phase plane representation of the action potentials shown in B. Note monophasic rate of rise. Inset, higher magnification of the action potentials onset shows the rapid onset. Grey lines show the linear fits used to calculate the onset speed of the action potentials. E, Plot of the action potential onset speed (slope of the initial rate of rise after the threshold (20 mV/ms) in the phase plane) versus voltage threshold. Line is the best-fit line by the equation y(onset speed) = 5.34 mV−1ms−1 – 0.26*x(Vm), n = 1722 spikes, r = −0.438, p<0.0001. Scale bars: A, vertical 20 mV, horizontal 0.5 s; inset, vertical 20 mV, horizontal 20 ms; B, vertical 20 mV, horizontal 0.5 ms; inset, vertical 10 mV, horizontal 0.5 ms; C, vertical 100 mV/ms, horizontal 0.3 ms. Some portions of raw data (intracellular voltage recordings) from the cell in this figure have been used in previous publications (see Methods).
Figure 8.
The onset speed of action potentials depends on the effective density of available sodium channels.
A, In vivo intracellular low-threshold burst discharges of a TRN neuron recorded during the first 10 minutes after impalement with a recording pipette loaded with QX-314 (0.5 M). Inset, typical episode of spindle oscillations from the same cell (pentobarbital anaesthesia). B, Sample of consecutive action potentials (n = 10) from the cell shown in A. C, Phase plane representation of the action potentials shown in B. D, Same cell as in A, recorded 25 minutes after impalement. Inset, example episode of spindle oscillations. E, Sample of consecutive action potentials (n = 10) recorded 25 minutes after impalement. F, Phase plane representation of the action potentials shown in E. G, Plot of the onset speed of action potentials versus the voltage threshold for the cell shown in A–F during the first 10 minutes of recording (early QX-314, n = 212 spikes) and after 25 minutes of recording (late QX-314, n = 254 spikes). H, Plot of the mean onset speed of action potentials versus the mean voltage threshold for each of the cells recorded with QX-314. Changes were statistically significant in all cells (p<0.05, two-tailed paired two-tailed t test). I, Onset speed of action potentials for all cells recorded in this study. Circles and grey bars show individual values and means, respectively. Control, data from neurons recorded without QX-314; early QX-314, data from neurons during the first 10 minutes of recording with QX-314; late QX-314, data from neurons after 25–50 minutes of recording with QX-314. Control, n = 23 cells, 32576 spikes; early QX-314, n = 4 cells, 1728 spikes; late QX-314, n = 4 cells, 386 spikes. Mean values were significantly different (*, p = 0.0004, Kruskal-Wallis test). Scale bars: A,D, vertical 20 mV, 50 ms; inset, vertical 20 mV, 0.5 s; B, E, vertical 20 mV, 0.5 ms. Some portions of raw data (intracellular voltage recordings) from the cell in this figure have been used in previous publications (see Methods).